The Romulan ship is using 'simple impulse power only' and yet crossed vast distances during the episode.

In this episode Spock declares cast rodinium to be the hardest substance known to Federation science. Yet in "Arena", Kirk claims that diamonds are "perhaps the hardest substance known in the universe".

Great Moment :

Virtually the entire episode.

Body Count :

Several hundred, most likely, on the Federation outposts and on the Romulan ship.

Factoid :

This episode introduces the Romulans, who will remain an adversary to the Federation for many years to come.

Phasers are used here for a "depth charge" style of weapon. The photon torpedoes would be a better match for this kind of weapon, but at this point in Trek history nobody had thought to invent them yet.

The Romulan crew wear helmets to avoid the cost of having to fit the whole crew with the pointed ears.

Plotline

The Enterprise is heading for the Romulan Neutral Zone, a buffer between the Federation and the Romulan Empire. Two outposts alongside the zone have gone silent, and the ship is tasked with investigating. Kirk is using some free time for a happy duty - officiating at a wedding between Angela Martine and Robert Tomlinson, two of the crew. However, the ceremony is interrupted by a red alert; Outpost 4 has signalled that it is under attack. Kirk races to the scene at high warp, whilst Spock briefs the crew on the situation. The Romulan Neutral Zone was established at the end of the Earth-Romulan War, more than a hundred years ago. Tresspass into the Zone by ships of either side is cause for war, according to the treaty. The Romulans are a mystery to the Federation, as there has never been visual communication with them and neither side took any prisoners in the war. Nobody has any idea what the Romulans look like, or what their attitude to the Federation might be.

Lieutenant Stiles comments on the situation repeatedly, stating that his family lost several people in the war with the Romulans. He seems tense and angry, and Kirk has to remind him that the conflict is far in the past and that their orders are to avoid combat if they reasonably can.

They manage to contact Outpost 4, finding it in ruins. An injured survivor reports that the attacker was invisible until the moment it fired, making them helpless against it. Additionally, the ship used some sort of new weapon which had incredible power. The ship reappears and fires again, destroying the outpost. Spock suggests that the enemy have a shield that can bend light around it, making it invisible to the eye. Sensors do manage to detect a rough trace on the ship, which is cruising quite slowly. Kirk wonders if the cloak system cuts both ways, making it difficult for them to see the Enterprise. He parallels the enemy vessel as it heads towards the Neutral Zone - the exact course a Romulan ship would be expected to take. Stiles is agitated by the caution Kirk shows, wanting to attack immediately.

Uhura intercepts a communication signal from the ship, allowing them to eavesdrop on the internal sensor systems and get a view of the bridge. To everyone's surprise the aliens are shown to be identical to Vulcans in appearance, prompting an incredulous reaction from Stiles in particular. When Uhura reports that the Romulan communications are in code he nastily comments that they should give it to Spock, prompting Kirk to reproach him for his behaviour.

On the Romulan ship the Commander detects a sensor contact behind him. Although his crew suggest that it is merely a sensor echo he is suspicious. He talks about his mission with a friend, Centurion. Their ship was to test the effectiveness of their new cloaking device and weapons in combat. Now that they have evidence of the Federation's weakness, they will return home only to see another war begin. The Commander is obviously saddened by this, but determined to do his duty regardless. He rebukes a subordinate, Decius, for sending the coded message without orders, prompting Centurion to comment that Decius has powerful political friends back home who could hurt the Commander's career.

On the Enterprise the officers discuss the situation. Analysis of the outpost debris shows that the entire asteroid has been pulverised, apparently by some sort of enveloping high energy plasma. Stiles again presses for an attack, arguing that the ship is still in their space so there would be no doubt of who was the aggressor. He argues that if the ship is able to get away it will show further weakness on their part, ensuring war. Sulu comments that being able to report the they had also destroyed the Enterprise would hardly improve the situation. Stiles is openly contemptuous of Spock, practically accusing him of hiding information about the Romulans. To his surprise Spock quietly agrees that an attack is the logical course of action, stating that if the Romulans are some lost offshoot of the Vulcan race then they are still ruled by their passions and emotions, and so extremely dangerous. McCoy argues for peace, but Kirk ultimately decides to make an attack.

The Romulan ship is on course to pass through the tail of a comet. Kirk suggests that the disturbance of the particles in the tail will make it visible, allowing the Enterprise to get a clear shot. He plans to circle around and lay in wait for them. However, the Romulan Commander comes to the same conclusion, and realises that the tail will also blind the Enterprise sensors for a short time. He is thus able to double back and evade the starship. Kirk is frustrated as having been out-thought, promising that he won't underestimate him again. he has the Enterprise fire phaser pulses across the whole area, blanketing everything in an attempt to score a lucky hit. The Romulan ship is damaged, killing the Centurion. As a malfunction shuts down the Enterprise phasers, the enemy Commander turns and fires. Kirk backs off at warp speed, watching the huge plasma bolt begin to disperse and weaken. nevertheless, when it hits the ship it results in a powerful blast that rocks the whole vessel. With their systems repaired the Enterprise blankets the area with more phaser fire, but the Romulan ship slips into the Neutral Zone. Kirk decides to chase it into the Zone, regardless of the risk of war.

The Romulan commander lays a trap for Kirk; he jettisons debris from the ship, including the body of his friend Centurion. The plan doesn't work as there is clearly not enough debris for a full ship. Both the Enterprise and Romulan ship power down and play a waiting game. Spock notes that the firing circuits need more work, and begins to carry out the repairs. He accidentally activates a panel, emitting a signal that the Romulan ship detects - an action that appalls Stiles and obviously makes him think it might be deliberate. The Romulan commander closes in to fire, but Kirk manages to open fire first, rocking his ship. He orders more debris released, but this time includes a small nuclear bomb. The plan works almost perfectly, with the Enterprise closing in and only detecting the weapons when it is very close. Kirk orders it detonated with phasers before it can impact the ship, but the blast leaves the Enterprise all but dead in space.

The Romulan commander sees his enemy helpless before him, but is reluctant to fire and destroy them. He argues that they are very low on fuel and need enough to get home, but ultimately decides to attack and advances on the ship.

On the Enterprise Kirk sends Stiles to man one of the phaser rooms, replacing injured crewmen. He plays dead again, trying to lure the enemy in. As Spock checks on the phaser room Stiles is again openly rude, dismissing him - but after Spock leaves a coolant system begins leaking poison gas. As the Romulan approaches Kirk desperately signals the room to fire. Spock, hearing this, rushes back and fires the weapon before dragging Stiles to safety. The Romulan ship takes a direct hit and is badly damaged.

Kirk manages to establish visual communication, offering to beam the survivors aboard, but the Romulan Commander says that this is not their way. He laments that circumstance has made he and Kirk enemies, when "in a different reality I could have called you friend". But he has one more duty to perform - tripping the self destruct switch that obliterates his ship and crew.

In sickbay, Siles is astonished that Spock saved his life in spite of his poor attitude. Unfortunately the other man in the phaser room was killed - Tomlinson, the man who was due to be married at the beginning of the episode. Kirk visits his former fiancé in the chapel and tries to console her as the episode draws to a close.

Analysis

One of the better original series episodes, Balance of Terror is a derivation of the old submarine movies set in World War II - and most closely, of a little gem called "The Enemy Below", which happens to be one of my favourite films. The episode scores high in many respects; making the Romulan ship decidedly different from the Enterprise in how it operates and fights means that both can be shown to play to their relative strengths, with victory going to the man who employs his ship most effectively. It makes the battle very much one of wits rather than, as is so often the case in science fiction battles, a matter of "he who has the bigger ship and most guns batters the smaller into destruction". There's also a nice touch in that the Enterprise is more than once limited by equipment failures. Seriously, we all know that equipment can and does fail from time to time, but how often does that happen in the movies? And then there's the intriguing prospect of Spock making an actual clear mistake, and a potentially fatal one at that. It all makes for a gripping episode.

Then we have Stiles. I rather like this aspect of the show, where we see an officer who is obviously competent but has something of an attitude problem. And you have to admit, for the most part he is right. He advocates an attack, and that's exactly what Kirk ends up doing. Indeed if they had attacked sooner, they might have prevented the Romulan ship from making it into the Neutral Zone at all. As for his attitude to Spock... well, you have to admit that finding that your XO is apparently of the same species as the enemy is at least cause to wonder. Apparently in the original script there were lines about how aspects of the Romulan ship seemed to be clearly based on stolen Federation technology, which made the prospect of a Romulan agent on the Enterprise even more likely. That said, Stiles obviously isn't motivated by these relatively sensible concerns, or at least not only by them. He's obviously coming from a place of anger and hatred in much of what he says, and his ultimate humbling is satisfying in this respect.

Where does the episode fall down? Really, the only note that hit me as being off is the whole wedding aspect. Really, we have a wedding service at the start only to kill off the groom at the end to inject a bit of pathos? It's rather over the top, IMO, especially coming on the heels of the more subtle sense of loss we get from seeing the Romulan Commander destroying himself.

As for the Romulans, it's a wonderful aspect of the episode to show Kirk up against what is virtually a mirror image of himself. The Romulan Commander is experienced, skilled, and very good at his job. Just not quite good enough, in the end. Or was he? One of the things I always took away from this episode was the possibility that he decided to engage in that last attack knowing it would fail, knowing it would mean victory for the enemy - and in doing so, knowing that it would mean that there would be no war after all. Can we say for sure? No... but watch the episode with this in mind, and you get a whole new layer to it.